Zeus 75 Power Amplifier
Zeus 75 Prototype Amplifier & PSU
Amplifier Schematic -
Click here for PDF of schematic for
N.B. Prototype uses
L200 bias current source, which has better stability.
Standard components for the amplifier
R1-2 are 470 ohm non-inductive 1/4 watt
resistors (note: do NOT use lower ohmic values, particularly for low Crss
R3 is selected by tuning first with a 470K
pot depending on the transformer specification, then is replaced with a fixed
non-inductive 1/4 watt resistor. With the
Sowter input transformer and a driving
source impedance of c. 50 ohms the value of RTerm is
Z1-2 are 12V Zeners to protect Q1-2 These
MUST be fitted as shown otherwise the mosfet gates can be driven over range and
the mosfets WILL be destroyed if this happens.
Fit Q3 to the heatsink near Q1 and Q2. Also
fit LM317 to heatsink to use it's thermal shutdown feature (note v-max for this
part - use a pre regulator or separate lower voltage power supply if the main
supply is going to be higher than 34 volts).
Alternative is a ST part type L200 regulator in current mode.
Note [Jan 2005] the L200 has much better regulation against power supply
variation then the LM317, and is to be preferred.
Over Temperature Protection
Also mount a normally open thermal switch
(not shown on the schematic) on the heatsink so arranged as to short out the
Schottky diode and thus reduce the bias current at a predetermined temperature (50°
There are several different options which can be built, most of which come down
to the required power output as determined by speaker efficiency and listening
My original mono-blocks are low quiescent current amplifiers for 12 or 35 watts
which can be constructed with modest specification components and a relatively
small heatsink. The choice of power output and distortion can be traded by the
use of a 4:1 or 2:1 winding configuration.
A high quiescent current amplifier which will give reduced distortion for the
higher power outputs (e.g. 75 watts into 8 ohms), will need a slightly larger
power supply but a substantially bigger heatsink, is detailed here.
N.B. For higher bias
currents it is recommended to use a tracking bias voltage supply as detailed
above, rather than a simple regulated voltage and a potentiometer.
Rendered Image of Zeus 75 Amplifier (only major
Mono-block Amplifier with separate PSU in its own steel
enclosure (this cuts down on the magnetic field radiation/pickup).
The amp is designed to be tall and not deep so as to sit behind or to one side
of the speaker it is driving.
Illustrated with an Aavid Thermalloy
OS245, size 300 x 200 x 80 mm (note 250 mm is recommended and used in the
Steel chassis is 300 x 150 x 80 mm which sits in front of and at
the bottom of the heatsink. The output transformer sits on top of the steel
chassis, the input transformer is inside and thus screened from electrical and
magnetic fields (including the output transformer's).
The PSU is in a separate 300 x 200 x 80 mm enclosure and sits under or to one
side of the amplifier. Power connection (not shown in this render) is a short
loop of cable with 4 pin Cliff connectors.
Chassis is 1.5mm steel.
Binding posts for 2:1 or 4:1 output. Heatsink is 250mm high.
Heatsink is mounted away from chassis with washers.
Transformer leads left long for testing.
Bias using ST L200 in current regulator mode (6R8 series resistor for 100
mA), a pair of Schottky diodes and third mosfet. Potentiometer across both
diodes for adjusting bias.
Power mosfets mounted either side of the bias temperature
tracking mosfet, L200 on right. Thermal switch on left shorts out lower diode
and reduces bias level to lower quiescent power being dissipated.
A second thermal switch with a higher temperature limit could be
used across both diodes to bring bias down to a minimum - which should be
discernable to the ear.
PowerMESH MOSFETs, mounted on a
heatsink of 300 x 250 mm with 80 mm fins MINIMUM.
(See http://www.st.com if above link does not
STW34NB20: N-Channel 200V; 0.062Ω; 34A;
Crss: Reverse Transfer Capacitance = 90 pF
The bias voltage is generated from a
temperature tracking supply using an additional MOSFET, schottky diode and
constant current source (e.g. LM317 or L200).
Bias is set for between 500 mA to 750 mA per
device - dependant on the size of the heatsink being used, the DC supply
voltage, and the likely peak ambient temperature in use.
Match MOSFETS as well as possible for
closest voltage drop across Drain-Source. Use a variation of the bias generator
for this. See: Matching MOSFETS page for
schematic and picture.
This is a separate unit to minimise magnetic interaction between
PSU and Amplifier.
Note the relatively small reservoir capacitor. Larger or
multiple capacitors may be used as per normal power amplifier practise but this
is not required for the Zeus push-pull power stage circuit arrangement.
Internal view of Assembly.
Specification of unit shown:
- Transformer 250 VA, 12+12 Vac; e.g.
Farnell # 9530720.
- Main Supply Reservoir Capacitor 10,000
uF (105 deg C), e.g.
Farnell # 4704939.
- Secondary Supply Reservoir Capacitor
- Cliff 4 pole power connector.
- Chassis is Schroff 1.5mm steel.
I have included the binding posts for
development use. For normal use the power supply is via the 4 way power
- Main Supply = 36 volts (off load),
fused 5A slow-blow.
- Secondary Supply = 17 volts, fused
For 75 Watts (in 2:1 mode) into 8 ohms the
power supply needs to be about 40 volts DC, which is above the direct voltage
rating of the LM317 regulators and at the maximum for L200 regulators. Using a
15+15 Vac transformer with the centre tap method derives a safe half volts
supply for the bias circuits.
An adequate size of heatsink is very important to ensure
adequate thermal stability and dissipation for any power amplifier.
Note: Please ensure that the maximum temperature that the
heatsink attains during operation at is 50°
Centigrade maximum. Above this temperature there is a risk of burns.
OS245, black anodized.
Use 250 mm length for Zeus 75 as illustrated in above
Thermalloy sales locations here.
Alternative Heatsink Profiles
Any large heatsink (assembly) should be fine as long as it is
capable of 0.3°C/W or less.
Suggested minimum Heatsink Size/Type for High Bias Current
Amplifier - 300 x 200 x 80 mm.
Illustrated above is a type SK158 profile heatsink from
DAU Components Ltd (rendered from
profile with TurboCAD V10 Pro) (This is the DAU equivalent to OS245). N.B. 200
is the minimum recommended size for high bias versions of the amplifier, 250 mm
(or greater) is preferred.
Good thermal contact between the semiconductor device and the
heatsink is very important to ensure proper operation and longevity of the power
My recommendation is
Bergquist Sil-Pad1500ST which has excellent characteristics and has good
performance with moderate contact pressure so is well suited to hand assembly.
Design by: Susan Parker, MIET.
The information contained here may be used to
construct one system specifically for personal NON commercial use only.
N.B. Personal liability disclaimer applies.